1. Field of the Invention
The present invention relates to a double-sided exposure system. More specifically, the present invention relates to a double-sided exposure system capable of sequentially exposing the first and second surfaces of a plate, such as a substrate for a printed wiring board or a sheet for lead frames, through an exposure mask provided with a predetermined exposure pattern to light, and, particularly, provided with a single exposure light source.
2. Description of the Related Art
Generally, an exposure system employed in, for example, a process for fabricating a high-density printed wiring board is provided with an extra-high pressure mercury lamp as an exposure light source for high resolution. The mercury lamp of such a type is a very expensive lamp costing as much as about .Yen.500,000. The process requires the mercury lamp to be kept continuously lighted and the life of the mercury lamp is only about 500 hr. Therefore, the exposure system of this type must be provided, if possible, with a single light source and the light source must be used at the highest possible economic efficiency; that is, it is desired to reduce the exposure cost of each substrate for a printed wiring board (the running cost of the light source) to the lowest possible extent by using the light source for as many exposure cycles as possible in its life.
To enhance the economic efficiency of the light source, it is important to enhance the operating speeds of mechanical units including a carrying mechanism for carrying substrates and alignment adjusting mechanism for aligning the substrates with an exposure mask and to reduce waiting time, such as exposure waiting time, near to naught.
Referring to FIG. 13 showing a double-sided exposure system 100 disclosed in Japanese Patent Application No. 343971/1997 (JP-A No. 333337/1998), a work holding base 101 for detachably holding a substrate P has opposite surfaces respectively provided with vacuum pads 103, and the work holding base 101 is moved repeatedly between a home position, i.e., a position indicated by chain lines, between a work receiving unit 105 and a work delivering unit 107, and an exposure position, i.e., a position indicated by solid lines, between two mask holding mechanisms 109L and 109R respectively holding exposure masks 113.
Light emitted by a mercury-short-arc lamp 111, i.e., a light source, is transmitted through a left optical path and falls on the back surface of the exposure mask 113 held by the left mask holding mechanism 109L, or the light is transmitted through a right optical path and falls on the back surface of the exposure mask 113 held by the right mask holding mechanism 109R. An optical path selecting unit, not shown, connects the mercury-short-arc lamp selectively to the left optical path or the right optical path. The work receiving unit 105 adjusts the position of an unexposed substrate P fed thereto for preparatory positioning and transfers the substrate P to the vacuum pad 103 on the left surface, as viewed in FIG. 13, of the work holding base 101 located at the home position. Upon the arrival of the work holding base 101 holding the unexposed substrate P on its left surface at the exposure position, the left mask holding mechanism 109L advances to bring the exposure mask 113 into contact with the substrate P so that the exposure mask 113 is aligned with the substrate P. After the exposure mask 113 has been exactly aligned with the substrate P, the exposure mask 113 is contacted fixedly to the substrate P. Then an exposure cycle is executed to expose a first surface, i.e., one of the surfaces, of the substrate P through the exposure mask 113 to light.
After the exposure cycle has been completed, the left mask holding mechanism 109L is removed from the substrate P, the left mask holding mechanism 109L is retracted, a left transfer hand 115L holds the substrate P and transfers the same to a right transfer hand 115R. Meanwhile, the work holding base 101 is returned to the home position, and then moved again to the exposure position. Subsequently, the right transfer hand 115R transfers the substrate P to the right vacuum pad 103, the right mask holding mechanism 109R is advanced into contact with the substrate P so that the exposure mask 113 is aligned with the substrate P. After the exposure mask 113 has been exactly aligned with the substrate P, the exposure mask 113 is contacted fixedly to the substrate P. Then an exposure cycle is executed to expose a second surface, i.e., the other surface, of the substrate P through the exposure mask 113 to light. Thus, the exposure of both the surfaces of the substrate P is completed.
After the completion of the exposure cycle, the right mask holding mechanism 109R is removed from the substrate P, the right mask holding mechanism 109R is retracted and the work holding base 101 is returned to the home position. Then, the left vacuum pad 103 receives an unexposed substrate P from the work receiving unit 105, and the work delivering unit 107 receives the two-side exposed substrate from the right vacuum pad 103 and delivers the same.
FIG. 14A is a time chart representing sequential operations of the prior art double-sided exposure system 100. In FIG. 14A, time is measured in seconds on the horizontal axis. Generally, exposure time is about 3 s or below. In this time chart, the exposure time is 3 s, and cycle time, i.e., time need to carry out a sequence of operations from the start of reception of an unexposed substrate P to the start of reception of the next unexposed substrate P, is 22 s. As obvious from FIG. 14A, there is a scarce waiting time between the completion of the exposure of the first surface of the substrate P and the start of the exposure of the second surface of the substrate P when the exposure time of the double-sided exposure system 100 is on the order of 3 s.
Some resist that is applied to the substrate P requires a longer exposure time. When a long exposure time is necessary, an exposure operation for exposing the second surface of the substrate P to light cannot be started before the completion of an exposure operation for exposing the first surface of the substrate P to light even if preparatory operations of the mechanical units for exposure are completed, which produces a time loss.
FIG. 14B is a time chart representing sequential operations of the prior art double-sided exposure system 100, in which exposure time is 8 s for both the surfaces of the substrate P. In this case, there is a waiting time of 5.5 s between the completion of preparations for the exposure of the second surface of the substrate P to light and the start of the exposure operation for exposing the second surface to light, and there is a waiting time of 5.5 s between the completion of transfer of the substrate P from the left side to the right side of the work holding base 101 and the start of shifting the work holding base 101 to the home position, i.e., during operations for exposing the second surface of the substrate P to light and post-operations. Consequently, the cycle time is as long as 32 s, which is about 1.5 times the cycle time needed by the operations shown in FIG. 14A and hence the running cost of the lamp 111 increases sharply near to 1.5 times the running cost needed by the operations shown in FIG. 14A.